Mechanical assemblies often use fasteners and typically blind rivets to secure one or more components together in a permanent construction. Blind rivets are preferred where the operator cannot see the blind side of the workpiece for instance where the rivet is used to secure a secondary component to a hollow box section. Also they are preferred where a high volume of assemblies are being produced as there are advantages to be gained from increased assembly speeds and productivity compared with say threaded or bolted joints.
One of the disadvantages of a blind rivet setting to a hollow box section is that the blind side set end of the rivet cannot be visually inspected for a correctly completed joint. This is especially relevant where there are a number of blind rivets used and these are of a multiplicity of different sizes both in diameters and lengths. Also there could be occasions where assembly operators are inexperienced or the arrangements of rivets are complex. Further, it is possible that rivets are incorrectly installed or perhaps not installed at all. To inspect assemblies after completion is not only expensive and unproductive and in some instances it is virtually impossible to identify if the correct rivet has been used in a particular hole. A further consideration can be that modern assembly plants are using increasing numbers of automative rivet placement and setting tools where there is an absence of the operator.
The current monitoring of a rivet during the setting process has been limited to the use of two methods. The first method employs the use of a hydraulic pressure transducer which measures working fluid pressure within the tool. This current method is limited to use in detecting fluid pressure alone. The second method uses a “load cell” mounted linear to the tool housing. This option used equipment which is considerably larger in size and has limited field capability as a result. Typically, the second method additionally uses a LVDT to measure the translations of the various moving components.
In accordance with the present invention, a system is provided that will continually monitor the setting process, the numbers of rivets set and the correctness of setting and to identify if there are small but unacceptable variations in rivet body length or application thickness. Also, because assembly speeds are increasing, it is an advantage to identify incorrect setting almost immediately instead of a relatively long delay where complex analysis of rivet setting curves is used. Other fasteners such as blind rivet nuts (POP®nuts), self drilling self tapping screws or even specialty fasteners such as POP®bolts can be monitored but for the purposes of this invention blind rivets are referred to as being typical of fasteners used with this monitoring system.
To overcome the disadvantages of the prior art, a rivet monitoring system is provided which has a micro-strain sensor that measures strains within a tool component. These measured strains are compared to a number of tolerance bands formed about median strain or pressure versus time curve. Various techniques are provided to analyze the measured data with respect to the tolerance bands to determine if a particular river set is acceptable. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.